This invention is related to color CRT display systems, in particular, those of the "self-converged" or "self-converging" types. These are systems in which the three electron guns, one each for red, blue and green picture information, are arranged horizontally "in-line". The deflection yoke has, in addition to the main deflection field components, an additional quadrupolar component which maintains the beams in convergence as they are deflected across the screen, without the need for dynamic convergence circuitry.
The theory and construction of self-converged type CRT color display systems are well-known and are described in the literature. For example, see U.S. Pat. No. 3,800,176.
Display systems of the self-converged type permit the use of greatly simplified convergence apparatus, and thus have the advantage of substantially reduced cost. Self-converged systems do, however, have the drawback that the same astigmatic yoke field component which so advantageously self-converges the beams, unfortunately produces rather severe deflection defocusing of the electron beams at the sides of the CRT screen. One of the effects of this defocusing appears as horizontal elongation of the beam spots.
Various approaches have been taken to reduce the real or apparent effects of this deflection defocusing of the beams at the screen edges. One approach is described in U.S. Pat. No. 3,984,723. It involves the provision of vertically oriented elliptical apertures in the G.sub.2 electrode of the gun. By causing the beam to have a vertically elliptical shape at the screen center, that is, a shape which is orthogonal to the horizontal beam deflection defocusing produced at the screen edges by the astigmatic yoke field components, some compensation for the deflection defocusing results. There are a number of drawbacks to this approach, however. First, it is believed that the amount and perhaps even the direction of the ellipticity induced in the beam changes as a function of beam current. Secondly, the gun is apt to be incapable of being standardized for a range of tube sizes, being limited for a given design to a particular CRT size and configuration. Another example of this approach is found in U.S. Pat. No. 3,881,136.
Thirdly, it is known that any gun having apertures which are not round is difficult to assemble. The conventional method for assembling and precisely aligning electron gun parts involves stacking them on rod-like mandrels and then joining the parts together by the use of molten glass rods. Any part having a noncylindrical hole cannot be precisely aligned on such a rod-like mandrel and thus is difficult to align with respect to the other parts.
Another approach involves forming a round beam in the lower end of the gun as is conventional. In the main focus lens of the gun a quadrupolar astigmatic field component is formed which introduces a vertical elongation of the beams at the screen center. The vertical elongation at least partially compensates for deflection defocusing of the beams.
This latter technique is employed in a non-standard color CRT display system in which three electron guns are arranged to share a common main focus lens. A dynamic quadrupolar magnetic field is established in the main lens which rounds out the beams. This system is described in "25-Inch 114 Degree Trinitron Color Picture Tube And Associated New Developments" by Sony Corporation, IEEE Spring Conference on BTR, June 10, 1974.
This latter-described system offers the advantage of producing no astigmatism in the beam when the yoke field is zero, that is, when the beams are in the center of the screen. It has the disadvantage, however, that in rounding out the beams at the edges of the screen, the size of the beam spots are undesirably increased. It has been found to be necessary in such a system to use dynamic focusing along with the deflection defocusing compensation in order to minimize the spot enlargement at the screen edges. Dynamic focusing is normally not needed in modern day color television receivers.
Yet another approach is described in U.S. Pat. No. 4,086,513-Evans. Evans discloses the use of axial extensions from certain gun electrodes in the region adjacent the beam-passing apertures. The extensions affect the beam-influencing electrostatic fields in such a way as to vertically elongate the electron beams. The vertical elongation is intended to compensate for deflection defocusing of the electron beams.